Fluorescence microscopic characterization of ionic polymer bead-supported
phospholipid bilayer membrane systems
ABSTRACT
Supported phospholipid membrane structures on cationic organic polymer
beads were prepared using mixtures of dioleoylphosphatidylserine (PS) and
egg yolk phosphatidylcholine (PC). Confocal fluorescence microscopic observations
using a fluorescent membrane probe (N-4-nitrobenzo-2-oxa-1,3-diazole-phosphatidylethanolamine)
revealed that the phospholipid molecules in the PS/PC-bead complexes were
along the outer surface of the beads, but not inside the beads. The anionic
PS on the most outer surface of the PS/PC-bead complexes was responsible
for the binding of a positively charged macromolecule, rhodamine isothiocyanate
dextran (Mw 70,000) by electrostatic attractive forces. The fluidity of
the membranes in the PS/PC-bead complexes was investigated by the fluorescence
recovery after a photobleaching technique. The lateral diffusion coefficients
(D) for the PS/PC-bead complexes were one-half or less than that for 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine
giant unilamellar vesicles without solid supporting materials. Such a constrain
of the phospholipid bilayer membrane in the complexes appeared to be due
to its immobilization on the cationic polymer bead by electrostatic attractive
forces between the PS and ammonium group on the surface of the bead. The
D values for the complexes were dependent on the phospholipid composition;
the PS(25 mol%)/PC(75 mol%)-bead complex produced a more fluid membrane
than the PS(50 mol%)/PC(50 mol%)-bead one. Thus, the fluidity of the phospholipid
bilayer membranes formed on the cationic polymer beads was significantly
affected by the anionic phospholipid fraction used for the preparation
of the complexes.